A U.S. security institute that advises Washington policy makers says Iran could produce enough weapons-grade uranium for a nuclear bomb in as little as two months from now, but would need additional time to make the device itself.

In a report issued Monday, the Institute for Science and International Security (ISIS) said Iran has the capacity to produce 25 kilograms of highly enriched uranium needed for the core of a nuclear warhead in two to four months. Iran has thousands of centrifuges enriching uranium at its main plant in Natanz and hundreds of other centrifuges operating at its Fordo facility, located under a mountain to shield it from air attacks.

The United States and Israel have refused to rule out military action to stop Iran from obtaining a nuclear weapon. Tehran says its nuclear program is designed only for peaceful uses.

The ISIS report did not say whether Iran has decided to convert its enriched uranium into a nuclear bomb. It said that if Iran were to attempt such a step, it would need more time to finish the job, "despite work it may have done in the past."

ISIS said Iran would need "many additional months to manufacture a nuclear device suitable for underground testing, and even longer to make a reliable warhead for a ballistic missile."

Israeli Prime Minister Benjamin Netanyahu said last month that Iran could reach the stage of producing weapons-grade uranium by the middle of next year, at current enrichment rates. In a speech to the U.N. General Assembly, he said that once Iran begins what he called the "final" enrichment stage, it would be only a few weeks or months until Iran has sufficient enriched uranium for its first bomb.

Netanyahu used the speech to reiterate his view that an Iranian advance to producing weapons-grade uranium is a "red line" that the international community must warn Tehran not to cross.

U.S. President Barack Obama told the General Assembly that the United States will do what it "must" to prevent Iran from obtaining a nuclear weapon. But he has refused to set a "red line" for Iran, insisting there still is time for diplomacy and sanctions to pressure Iran into stopping its enrichment work.